Color solid-state image sensors typically have a spectral sensitivity that differs from colors that are perceived by the human eye. Consequently, colors of an image perceived by a human eye tend to differ from colors generated by solid state image sensors. To improve the color reproduction accuracy, digital cameras with image sensors having Red, Green and Blue (R, G, B) output channels usually perform color correction with a 3×3 color correction matrix. Ideally, the matrix has matrix coefficients that are optimized to improve color fidelity. A method of adjusting matrix coefficients to improve color fidelity is described in published patent specification WO 00/10331. The method described may work well with Charged Coupled Device (CCD) sensors that have very low level of noise. However, CMOS image sensors inherently suffer from thermal noise and fixed pattern noise. Accordingly, this method of adjusting matrix coefficients is not ideally suited to CMOS image sensors as the coefficients tend to amplify noise thereby significantly affecting image quality.
The problem of noise amplification by color correction is reported in the publication “Performance Analysis of a Color CMOS Photogate Image Sensor”, IEEE Transactions on Electronic Devices Vol. 47, N1J January 2000. Noise performance is suggested to be resolved by improving the sensor quantum efficiency rather than by color correction methods.
In the publication “Design of real-time Image Enhancement Preprocessor for CMOS Image Sensor”, IEEE Transactions on Consumer Electronics, Vol.46 N1, 2000. Color correction hardware complexity is assessed rather than noise amplification problems. Complexity is an important issue especially if the color correction is to be implemented directly on the CMOS sensor chip. Nevertheless, the problem of noise affecting image quality is not addressed in this publication.
Another approach to reduce color correction complexity is proposed in “Color Signal Processing Technique for Single-Chip CCD Cameras That Employ CPUs with SIMD Instruction Sets”, IEEE Transactions on Consumer Electronics, Vol.46 N2, 2000. The color correction technique described operates directly on Color Filter Array samples. The technique is implemented as a vector operation and does not address noise amplification.